Earth's time bombs may have killed the dinosaurs

作者：漆烽征 发布时间：2019-03-01 05:19:01

By Colin Barras THE fate of the dinosaurs may have been sealed half a billion years before life even appeared, by two geological time bombs that still lurk near our planet’s core. A controversial new hypothesis links massive eruptions of lava that coincided with many of Earth’s largest extinctions to two unusually hot blobs of mantle 2800 kilometres beneath the crust. The blobs formed just after the Earth itself, 4.5 billion years ago. If the hypothesis is correct, they have sporadically burst through the planet’s crust, creating enormous oceans of lava which poisoned the atmosphere and wiped out entire branches of the tree of life. Enormous oceans of lava poisoned the atmosphere and wiped out entire branches of the tree of life Debates still rage over what caused different mass extinctions, including the one that wiped out the dinosaurs. An asteroid that smashed into Earth 65 million years ago is no doubt partially to blame for the Cretaceous giants’ demise. But a less-known school of thought has it that this and other extinctions occurred when cracks in the crust let huge amounts of lava gush from the bowels of the Earth. Each event flooded at least 100,000 square kilometres, leaving behind distinct geological regions known as large igneous provinces (LIPs), such as India’s Deccan traps, formed when the dinosaurs went extinct (see map). “There is an amazing correlation between mass extinctions and LIPs,” says Andrew Kerr at the University of Cardiff, UK. Now Matthew Jackson at Boston University, and colleagues, claim to have found evidence that LIPs are fed by 4.5-billion-year-old stores of mantle. Most of the mantle has been modified by plate tectonics since then (see “Diamonds and the birth of plate tectonics”). But last year Jackson’s team found that 62-million-year-old basalts from the North Atlantic LIP contain isotopes of helium, hafnium and lead in ratios that reflect the chemistry of early Earth’s mantle. They have now found similar lead isotope ratios in other LIP rocks, and say that LIPs in general may have an ancient source. Their analysis suggests this mantle contains an abundance of radioactive, heat-producing elements, making it unusually hot and potentially more likely to form the large quantities of lava needed to create LIPs (Nature, DOI: 10.1038/nature10326). The ancient stores might still exist. Studies to probe the mantle’s structure with seismic waves have revealed two unusual areas some 2800 kilometres down, beneath Africa and the Pacific Ocean. Trond Torsvik of the University of Oslo, Norway, and colleagues recently showed that most LIPs formed while one of these two areas lay directly beneath them. “It’s an interesting idea – that a giant blob of hot magma might burp from near Earth’s core every now and then, causing havoc for life,” says Gerta Keller at Princeton University, but adds more work is needed to support the hypothesis. Kerr agrees: “This will be controversial – it flies in the face of much of the research from the last 30 years.” Conventional wisdom, he points out, suggests LIPs have a more prosaic source – young mantle formed when oceanic crust returns to the mantle through subduction. But Torsvik is enthused. Having spent 10 years collecting evidence that his two mantle blobs have been stable for at least 540 million years, the idea that they contain primordial mantle is “like music to my ears”, he says. The inside of our planet is a magma-churning power house. As a result, very little remains from the millennia just after Earth formed. The two blobs of ancient magma that may be responsible for several mass extinctions are an exception. Some diamonds, it turns out, may also serve as time capsules. A new study suggests that locked inside them is the secret of when the continents formed. We knew that plate tectonics have been pushing new bits of crust into existence and engulfing old chunks back into the mantle for hundreds of millions of years. What we didn’t know is when it all started. Stephen Richardson at the University of Cape Town, South Africa, and Steven Shirey of the Carnegie Institution for Science in Washington DC collected thousands of ancient diamonds from around the world. Gems considered to be flawed by the jewellery industry can contain tiny clumps of minerals from the rocks in which they formed. Some clumps are made of peridotite, others of the rarer eclogite, which is only formed when volcanic rocks from the surface are forced deep into the mantle and crushed in the immense pressure and heat there. To get eclogite, you need plate tectonics. When Richardson and Shirey dated the mineral clumps, they found that the peridotite ranged from 2 to 3.5 billion years old, but the oldest eclogite was 3 billion years old (Science, DOI: 10.1126/science.1206275). This, say the researchers, proves that plate tectonics cannot have been active before then. Michael Marshall More on these topics: